Silica sulfuric acid-catalyzed one-pot synthesis of 1,3-diarylimidazolium tetrafluoroborate

Article abstract of DOI:10.14233/ajchem.2015.18954

A silica sulfuric acid-catalyzed three-component one-pot synthesis of 1,3-diarylimidazolium tetrafluoroborates was undertaken via the reaction of aromatic amine, formaldehyde and glyoxal at room temperature in ionic liquid [BMIM][BF₄] with good

Full text of DOI:10.14233/ajchem.2015.18954

Asian Journal of Chemistry; Vol. 27, No. 8 (2015), 3107-3110
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18954
Silica Sulfuric Acid-Catalyzed One-Pot Synthesis of 1,3-Diarylimidazolium Tetrafluoroborate
1,†
1
1
1
1
YU WAN2,*,†, WEN-LI ZHANG , HAO CUI , LI-ZHUO ZHANG , QIU-JU ZHOU , HUAN ZOU ,
1
1
1
2
1,2,*
YUAN-JIANG WANG , WEN-HUI GE , YUE FANG , SHENG-LIANG ZHOU and HUI WU
1School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, P.R. China
2State Key Laboratory Cultivation Construction Base of Biotechnology on Med-edible Plant of Jiangsu Province, Jiangsu Normal University,
Xuzhou 221116, P.R. China
*Corresponding author: Fax +86 516 83500164; Tel: +86 516 83403163; E-mail: wuhui72@126.com
†These authors contributed equally to this work.
Received: 23 December 2014;
Accepted: 7 February 2015;
Published online: 27 April 2015;
AJC-17202
A silica sulfuric acid-catalyzed three-component one-pot synthesis of 1,3-diarylimidazolium tetrafluoroborates was undertaken via the
reaction of aromatic amine, formaldehyde and glyoxal at room temperature in ionic liquid [BMIM][BF4] with good yields. Up to four new
bonds and one new ring were formed in one-pot with water as the only by-product in the reactions. The product 4d was determined by
X-ray diffraction. This work develops an efficient and low-cost preparation of 1,3-diarylimidazolium and N-heterocylic carbene.
Keywords: Synthesis, Silica sulfuric acid, 1,3-Diarylimidazolium tetrafluoroborate.
high yield and easily controlled reaction conditions. How-
ever, a long time reflux in organic solvent and absolute removal
of organic solvent were necessary to ensure the final cycli-
zation.
INTRODUCTION
Since the first isolation and characterization of N-hetero-
cyclic carbene(NHC) by Arduengo and co-workers1 in 1991,
various stable N-heterocyclic carbenes (NHCs)2,3 has been
investigated and applied extensively. It is possible to prepare
new nucleophilic reagent4, organic catalysts5, organic metal
catalysts6 and chiral catalysts7 from N-heterocyclic carbenes,
which promoted the revolutionary progress in the key fields
of organic synthesis and polymer chemistry8.
Imidazolium salt has been widely used in organic synthesis
as precursors. It was generating great interest in their usage as
versatile organocatalyst9-12, ionic liquid13 and key ligands14-19
in N-heterocyclic carbene complexes.Most of all, it represents
the typical architectures of stable nucleophilic singlet carbenes.
Accordingly, the development of simple and efficient strategies
for the construction of this molecular architecture is of consi-
derable importance.
EXPERIMENTAL
IR spectra were recorded with a Varian FTIR-Tensor-27
1
spectrophotometer using KBr optics. H NMR spectra were
recorded at 400 MHz on a Bruker DPX 400 spectrometer using
TMS as an internal standard and DMSO as solvent. Mass was
determined by using a Bruker TOF-MS high resolution mass
spectrometer. All reagents were obtained from commercial
suppliers and used without further purification unless otherwise
stated. Silica sulfuric acid was prepared in our lab. Organic
solvents were dried and distilled prior to use.
Experimental process
Synthesis of 1,3-diarylimidazolium tetrafluoroborate:
A mixture of amines (2.0 mmol), formaldehyde (1.0 mmol),
glyoxal (1.0 mmol) and silica sulfuric acid (0.2 g) and
[BMIM]BF4 (3.0 mL) was stirred at room temperature for 1.5 h
until complete consumption of the starting material as monitored
by TLC. After completion of the reaction, the mixture was
diluted with water; the crude solid was recrystallized with 95 %
EtOH/DMF (1:4), and then filtered to remove silica sulfuric
acid to provide the pure product 4.
Generally, imidazolium salts were prepared by two steps
involving the condensation of glyoxal, amine and formalde-
hyde and then the dehydration in the presence of strong acid20.
However, these procedures must be processed under some
harsh reaction conditions (high reaction temperature, strong
acid and so on) with low total yields. Comparatively speaking,
their one-step synthesis by condensation of 1,2-diamine with
formaldehyde or ammonium formate or triethyl orthoformate21
gradually became the main method with the advantages

3108 Wan et al.
Asian J. Chem.
Spectral data for compounds
1,3-Bis(3-chloro-4-fluorophenyl)imidazolidine (4i):
White crystal, m.p. > 300 °C, 0.36 g (81 %). H NMR (400
1
1,3-Di-p-tolylimidazolidine (4b): White crystal, m.p. >
1
MHz, DMSO-d6): δ 10.90 (s, 1H, ArH), 8.15-8.13 (m, 2H,
ArH), 8.08-8.06 (m, 2H, ArH), 7.85-7.83 (t, 3H), 7.39-7.26
(m, 2H, ArH). IR (cm-1): 3080, 2942, 1621, 1565, 1424, 926,
97, 765, 651. HRMS: calcd. for C15H12N2Cl2F2 [M+H]+ found
(expected): 328.0398 (328.0346).
300 °C, 0.29 g (85 %). H NMR (400 MHz, DMSO-d6): δ
11.80 (1H, q, ArH), 8.00-7.98 (4H, d, J = 8.4 Hz), 7.54 (s,
2H), 7.44-7.42 (4H, d, J = 8.0 Hz), 2.44 (s, 6H). IR (cm-1):
3441, 3340, 3924, 2861, 1606, 1557, 1452, 1381, 948, 722,
633. HRMS: calcd. for C17H20N2 [M+H]+ found (expected):
252.1678 (252.1626).
RESULTS AND DISCUSSION
1,3-Bis(4-methoxyphenyl)imidazolidine (4d): White
crystal, m.p. > 300 °C, 0.34 g (91 %). H NMR (400 MHz,
1
Herein we integrated the advantages of these two methods
mentioned above, reported a simple and efficient method for
the synthesis of 1,3-diarylimidazolium tetrafluoroborates via
the one-pot reaction of aromatic amine, formaldehyde and
glyoxal catalyzed by silica sulfuric acid at room temperature
in ionic liquid [BMIM][BF4] with good yields (Scheme-I).
Initially, to optimize reaction condition, some catalysts
(entries 1-5) were screened when p-methoxyaniline (1d), glyoxal
and formaldehyde, were chosen as substrates (Table-1). It was
shown that 0.2 g silica sulfuric acid is the most suitable one
(entry 5) which may attribute to its suitable acidity, whereas
catalysts possess stronger acidity or basicity have no advan-
tages in this reaction. Consequently, all further studies were
conducted using 0.2 g silica sulfuric acid as catalyst (entry 5).
Having established the best catalyst, the effect of solvents
(Table-1, entries 5-10) and time (entries 5 and 11-13) on the
model reaction was subsequently investigated (Table-1). The
results indicated that this reaction can not proceed in common
organic solvents except ionic liquids. [BMIM]BF4 gave the
highest yield (entry 5) than ethanol, THF, DMF, [BPY]BF4,
H2O and [BMIM]BF4. [BPY]BF4 can also promote the reaction
but in lower yield (entry 9) than [BMIM]BF4 due to its slight
weak acidity than that of [BPY]BF4. It was observed that the
suitable time was 1.5 h.
So, the mixture of aromatic amine (1), formaldehyde (2),
glyoxal (3) and [BMIM]BF4 was stirred in the presence of 0.2
g silica sulfuric acid at room temperature for 1.5 h to give the
corresponding compounds (4) (Table-2). In all the cases, 1,3-
diarylimidazolium tetrafluoroborates have been obtained in
moderate to high yields in short times (1.5 h). It was important
to note that the process tolerates both electrondonating groups
(such as alkoxy groups, Table-2, entries 3-4) and electron-
withdrawing substituents (such as halide groups, Table-2,
entries 5-9) on aromatic amine.
DMSO-d6): δ 10.15 (s, 1H, ArH), 8.46-8.45 (2H, d, J = 1.6
Hz,ArH), 7.83-7.816 (4H, d, J = 9.2 Hz,ArH), 7.25-7.23 (4H,
d, J = 8.8 Hz,ArH), 3.86 (6H, s). 13C NMR (100 MHz, DMSO-
d6): δ 160.16, 13.89, 127.78, 123.60, 121.955, 115.13, 55.78.
IR (cm-1): 3367, 3069, 2938, 2857, 1794, 1505, 985, 743, 679.
HRMS: calcd. for C17H20N2O2 [M+H]+ found (expected):
284.1583 (284.1525). C17H19N2O3Cl, monoclinic, P1, a =
15.6706 (19) Å, b = 9.4198 (9) Å, c = 5.0402 (64) Å, α =
90.00o, β = 90.1560 (10)o, γ = 90.00°, V = 797.50 Å3, Z = 2, R
(F) = 0.0294, wR (F2) = 0.0737, T = 298 K.
1,3-Bis(4-fluorophenyl)imidazolidine (4e): White
1
crystal, m.p. > 300 °C, 0.29 g (83 %). H NMR (400 MHz,
DMSO-d6): δ 10.29 (1H, s, CH), 8.53-8.52 (2H, d, J = 7.6
Hz), 7.99-7.95 (4H, d, Ar), 7.63-7.59 (4H, t). IR (cm-1): 3151,
3024, 1614, 1590, 1512, 1483758, 650. HRMS: calcd. for
C15H14N2F2 [M+H]+ found (expected): 260.1185 (260.1125).
1,3-Bis(4-chlorophenyl)imidazolidine (4f):White crystal,
m.p. > 300 °C, 0.31 g (82 %). 1H NMR (400 MHz, DMSO-d6):
δ 10.47 (s, 1H,ArH), 8.58-8.58 (2H, s,ArH), 8.00 (2H, s, ArH),
7.97 (2H, s, ArH), 7.83-7.80 (3H, t,ArH). 13C NMR (100 MHz,
DMSO-d6): δ 135.07, 134.61, 133.48, 130.15, 123.91, 121.97.
IR (cm-1): 3102, 1634, 1546, 1495, 763. HRMS: calcd. for
C15H15N2Cl2 [M+H]+ found (expected): 293.0669 (293.0607).
1,3-Bis(3-bromophenyl)imidazolidine (4g): White
1
crystal, m.p. > 300 °C, 0.37 g (79 %). H NMR (400 MHz,
DMSO-d6): δ 10.44 (s, 1H, ArH), 8.61 (s, 2H, ArH), 8.26 (s,
2H, ArH), 7.96-7.90 (s, 2H, ArH), 7.94-7.86 (s, 2H, ArH),
7.71-7.67 (t, 2H, ArH). 13C NMR (100 MHz, DMSO-d6):
δ135.71, 135.26, 132.90, 132.03, 124.86, 122.58, 121.85,
121.06. IR (cm-1): 3175, 3123, 1717, 1605, 1549, 1514, 635.
HRMS: calcd. for C15H14N2Br2 [M+H]+ found (expected):
379.9575 (379.9524).
1,3-Bis(4-bromophenyl)imidazolidine (4h): White
1
The structures of compound 4d (Fig. 1) was determined
by X-ray crystal structure analysis (CCDC 965289). The
ethanol (95 %)-dimethylformamidc (DMF) solution (10:1) of
the products stood at room temperature for 5-7 days to give
single crystals suitable for X-ray diffraction analysis. The
crystal data clearly indicated that the structures of products
conform to the speculated structures completely. There are
crystal, m.p. > 300 °C, 0.39 g (84 %). H NMR (400 MHz,
DMSO-d6): δ 10.39 (s, 1H, ArH), 8.58 (2H, s,ArH), 7.98 (2H,
s), 7.96 (2H, s, ArH), 7.89 (2H, s, ArH), 7.87 (2H, s, ArH). 13C
NMR (100 MHz, DMSO-d6): δ 134.95, 133.90, 133.07,
124.12, 123.08, 121.92. IR (cm-1): 1571, 1564, 1454, 1378,
1069, 751, 625. HRMS: calcd. for C15H14N2Br2 [M+H]+ found
(expected): 379.9573 (379.9524).
BF
4
O
SSA/r.t
N
N
H
NH
2
CH O
+
+
2
2
H
R
[BMIM]BF
4
R
R
O
1
2
3
4
Scheme-I: One-pot synthesis of 1,3-diarylimidazolium tetrafluoroborates

Vol. 27, No. 8 (2015)
Silica Sulfuric Acid-Catalyzed One-Pot Synthesis of 1,3-Diarylimidazolium Tetrafluoroborate 3109
TABLE-1
OPTIMIZATION OF THE REACTION CONDITIONSa
BF4
O
SSA/r.t
N
N
H
2 H3CO
NH2
+
CH2O +
H
[BMIM]BF4
OCH3
H3CO
O
1d
2
3
4d
Entry
1
2
3
4
5
6
7
8
Solvent
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
C2H5OH
THF
DMF
[BPY]BF4
H2O
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
[BMIM]BF4
Catalyst
HCl
NaOH
NaHSO3
–
Cat. (g)
Time (h)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1.0
2.0
Yieldb (%)
NRc
NR
NR
NR
91
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
0.4
0.2
0.2
0.2
Silica sulfuric acidd
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
Silica sulfuric acid
NR
NR
NR
87
0
50
85
70
33
80
9
10
11
12
13
14
15
16
91
aReactions were performed in 2:1:1 (p-methoxyaniline: glyoxal: formaldehyde) at room temperature in different conditions; Yield of isolated
b
products; c No reaction; d Prepared in our lab.
O
TABLE-2
H2
C
R
NH2
H
N
H
N
R
NH2
+
SYNTHESIS OF COMPOUND 4 CATALYZED
H
C
H
R
N
CH2
R
R
BY SILICA SULFURIC ACID a
A
B
O
C
O
C
Time
(h)
Yieldb
(%)
m.p.
(°C)
Entry
R
Product
H
H
C
H
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
87
85
76
91
83
82
79
84
81
> 300
> 300
> 300
> 300
> 300
> 300
> 300
> 300
> 300
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
4-CH3
2-CH3O
4-CH3O
4-F
4-Cl
3-Br
HO
OH
SSA
TM
E
N
N
R
R
D
Scheme-II
4-Br
3-Cl-4-F
The addition of (A) to nucleophiles R-NH2 then gave the inter-
mediate (B). Subsequently, the intermediate (B) condensed
with the glyoxal to form the intermediate (D). Then, the
dehydration and rearranging of (D) via [1,5]sigmatropic shift
to give the final 1,3-diarylimidazolium tetrafluoroborates (E).
In summary, we have developed a novel and simple silica
sulfuric acid-catalyzed three-component condensation for the
construction of 1,3-diarylimidazolium tetrafluoroborates from
commercially available starting materials. Silica sulfuric acid
showed its important role in this interesting reaction. Up to
four new bonds and one new ring were formed in one-pot
with water as the only by-product in the reactions. This method
offers several advantages including inexpensive starting
materials, low catalyst loading and no formation of by-products.
This work develops an efficient and low-cost preparation of
1,3-diarylimidazolium and N-heterocylic carbene.
aAll reactions were carried out in the scale of 0.2 g silica sulfuric acid
in 3 mL of [BMIM]BF4 at room temperature and starting materials
(1:2:3 = 2.0:1.0:1.0 mmol) were completely consumed; bIsolated yield.
Fig. 1. X-ray crystal structure of compound 4d
ACKNOWLEDGEMENTS
one water molecule and one chloride ion as impurities from
glass container in the single crystal structure of 4d.
The probable mechanism was shown in Scheme-II. The
first step involved the acid-catalyzed reaction of aromatic
amine (1) with formaldehyde (2), which acted as the source
of methylene to give imine (A) as the first key intermediate.
The authors are grateful to the foundation of the National
Natural Sciences Foundation of China (No. 31300067), Major
Project of Natural Science Research of University in Jiangsu
(No. 14KJA430003), Natural Sciences Foundation of Jiangsu
NormalUniversity(No. 13XLA01, 13XLA02)forfinancialsupport.

3110 Wan et al.
Asian J. Chem.
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